Broad bean chocolate spot

Chocolate spot is the commonest fungal disease of broad beans. Initially, it produces reddish-brown, round spots on all above ground parts of the plant. These become much larger, irregularly shaped and a chocolate brown colour, spreading further over the plant as the disease progresses throughout spring.

As the disease develops, the leaves shrivel and die, flowers are affected resulting in a poor or no crop, and severe infections on the stem can cause the plant to collapse.

The disease is more prominent during cool, humid weather – especially from late winter, on autumn-sown crops, to spring – and where plants are crowded too thickly together.

In gardens, chocolate spot only attacks broad beans – other beans are unaffected. Although it can attack bean species used as a green manure crop, such as winter field beans.

• Broad beans

Chocolate spot is caused by the fungi Botrytis fabae and Botrytis cinerea, which also causes grey mould on other plants.

They produce small, black resting structures (sclerotia), which can remain dormant in the soil for several years and keep the disease going from year to year even when broad bean plants aren’t unavailable.

They also produce masses of airborne spores, spreading the disease from plant to plant. Chocolate spot is also transmitted on infected seed.

During dry conditions, the disease remains as the small round spots, but under damp conditions it spreads rapidly across the plant and from plant to plant.

It is difficult to control chocolate spot once it is established. So check plants regularly, especially during wet or damp weather.

Don’t sow or grow plants too closely together to help maximise airflow around the plants. Avoid growing broad beans in damp, humid sites.

Removed affected leaves and destroy them as soon as symptoms are seen to help prevent the disease spreading. Badly affected plants should be carefully dug up, trying not to disturb any spores, and destroyed.

Don’t grow broad beans in the same soil for three years.

Unfortunately, there are no fungicides available for home gardeners to use to treat diseases on edible crops.

Don’t keep seed from affected plants for sowing the following year. Check young plants for the initial symptoms – small, round, reddish-brown spots – before buying.